Effect of visual adaptation upon intensity of illumination preferred for reading with direct lighting

"The illumination used was derived from standard metal shade, flexible arm reading desk lamps. Range of illumination available was from 4 to 100 foot-candles. Sixty subjects observed at two sittings. At one sitting they were adapted to 20 foot-candles; at the other, to 50 foot-candles. After adaptation, each subject chose between the standard to which he was adapted and each of five other intensities on the basis of the intensity preferred for easy and comfortable reading. Visual adaptation at time of choice influenced the choice only moderately. There was a marked tendency to prefer intensities at and above the brightness to which the subject was adapted. This resulted in frequent choice of high intensities. Since, in direct lighting such as used here, bright intensities make a bad situation worse from the viewpoint of hygienic vision, preferences for illumination intensities for reading yield unsatisfactory data for prescribing lighting for the individual."